A typical perlite mine

 

Origins and Characteristics.

Perlite is not a trade name but a generic term for a naturally occurring siliceous rock. The distinguishing feature that sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands from four to twenty times its original volume.

This expansion is due to the presence of two to six percent combined water in the crude perlite rock. Then quickly heated to above 1600 degrees F (871 degrees C), the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles that account for the amazing light weight and other exceptional physical properties of expanded perlite.

Three stages of perlite production shown above illustrate the great increase in volume after funacing.
The same weight of perlite, 1 oz. (28 gm.), is shown as crude perlite, crushed crude perlite, and expanded perlite.

The expansion process also creates one of perlite's most distinguishing characteristics: its white color. While the crude rock may range from transparent light gray to glossy black, the color of expanded perlite ranges from snowy white to grayish white.

Expanded perlite can be manufactured to weigh as little as 2 pounds per cubic foot (32 kg/m3) making it adaptable for numerous applications.

Since perlite is a form of natural glass, it is classified as chemically inert and has a pH of approximately 7.